Consumers are demanding increasingly sophisticated functionality from their mobile devices. For instance, the ability to have a video chat over a wireless network on a mobile phone is a sophisticated and complicated type of service mobile phones are expected to offer. The demand for increased functionality increases the complexity of the underlying circuitry of a mobile device and decreases the amount of space on the circuit board for various types of circuitry of the mobile device. One of the most complex and space-consuming types of circuitry in a mobile device is the signal processing circuitry. In particular, resonant circuits, within the signal processing circuitry, possess inductors, which are typically difficult to miniaturize or condense into smaller areas of a mobile device circuit board.
The difficulty in miniaturizing or condensing inductors is due to design limitations in achieving a high quality (Q) factor and a small coupling factor. The Q factor of an inductor is the ratio of the inductor's inductive reactance to its resistance at a given frequency, and is a measure of the inductor's efficiency. High internal resistances lower the Q factor of an inductor.
Inductor Q factors are commonly the limiting design factor for the insertion loss of passive filters and impedance matching circuits that are commonly found in front end modules, antenna tuners, tunable band pass filters, duplexers, and similar resonant circuits. Inductors used in these applications need to provide good isolation to avoid signal leakage. Isolation between current planar inductors is limited by a coupling factor resulting from the magnetic field generated across the design plane, as shown in FIG. 1. The magnetic field is open outside of an inductor 10, and without any field cancellation, the inductor 10 picks up the magnetic field of an inductor 12, and vice versa, increasing the coupling factor between the inductors 10 and 12.
One known method of solving the isolation design limitations presented in FIG. 1 is to simply widen the distance between the inductor 10 and the inductor 12 so the inductors 10 and 12 do not pick up each other's magnetic fields. This solution simply is not viable in resonant circuitry on mobile device circuit boards as the circuit board space is simply not available.
Another known method of solving the isolation problem shown in FIG. 1 is to “fold” the circular inductors 10 and 12 into a folded figure eight design. The coupling factor between the inductors 10 and 12 is reduced or improved, but the magnetic field still runs across the design plane, such that significant spacing is still needed between the inductors 10 and 12 and underpass circuitry connected to this known solution can be complex.
Still another known method of solving the isolation design limitations shown in FIG. 1 is to create a vertical coil inductor within a multi-layered substrate, such as a laminate, utilizing standard tube vias. Placing the coil inductor vertically within the multi-layered substrate, instead of horizontally as shown in FIG. 1, enables the magnetic field to run parallel to the design plane, reducing the coupling factor of the inductor. However, standard tube vias limit inductor performance. When placing multiple tube vias in parallel, to create a coil, the required spacing between standard tube vias limits the metal density of the inductor, limiting the Q factor. Also, the magnetic field of the inductor will cause the current to be restricted to a very small effective area of the standard tube vias, further limiting the Q factor.
Thus, there is need for a high Q factor vertical inductor with a small, or substantially zero, coupling factor that does not take up a significant amount of space on a circuit board of a mobile device.